1. Field of the Invention
This invention relates to radio frequency (RF) power amplifiers and in particular to wide bandwidth and high power radio frequency amplifiers.
2. Prior Art Solid State Power Amplifiers
Power amplifiers are the most functional-restrictive and cost-inhibit element within a communication system. More often than not, the output power parameter is achieved at the expense of bandwidth. In other words, output power and bandwidth are two parameters to be traded-off in the design of conventional power amplifiers. The reason for such compromise is due to the fact that, for example, a conventional solid-state power amplifier is normally comprised of many active devices combined in parallel to achieve the required power level. When many active devices are combined in parallel, the collective impedance decreases drastically. Subsequently, this combined and reduced impedance must be transformed back to the 50-Ohm system using an elaborate, often in a form of a multi-section topology, circuit which is either narrow band or large in size or both. Thus, conventional power amplifier designs often yield limited bandwidth.
Previously proposed power amplifier architectures employ non-stacked balun transformers, circular topology or series biasing technique to realize the power combiner/splitter circuitry. For example, a broadband balun and impedance transformer for push-pull amplifiers are described in Zhao et al., U.S. Pat. No. 6,819,200. A power splitter/combiner circuit, high power amplifier and balun circuit is described in Ishida et al., U.S. Pat. No. 6,803,837. A distributed circular geometry power amplifier architecture is described in Aoki et al., U.S. Pat. No. 6,737,948. A high-voltage series-biased FET amplifier for high-efficiency applications is described in Schellenberg, U.S. Pat. No. 6,163,220. These patents are incorporated herein by reference. However, these prior art amplifier architectures fall far short of realizing an amplifier architecture that can simultaneously achieve both very high output power (up to kilo-Watts) and extremely wide bandwidth (multi-octave or even decade).
For example, FIG. 2 shows the circuit schematic of a prior art power amplifier in which eight transistors are being combined in parallel, by means of a corporate (also know as ‘binary’) combining scheme. If the assumption is made that the terminal impedance of each transistor being deployed here is a real 4-Ohms, at either the input or output side, the combined impedance of eight parallel transistors will be 0.5-Ohm. Matching 0.5-Ohm to the system impedance of 50-Ohms at the ports, even over a modest bandwidth, is a significant challenge, if not impossible at times. Moreover, a corporate combiner must utilize several stages to accomplish the desired output power level. If the number of devices being combined can be expressed as 2n, then there will n number of stages needed thus given rise to the name ‘binary’ combining scheme. Total insertion loss becomes very significant as the result and it is the key factor that curtails the use of corporate combiner in applications in which both wide bandwidth and high power are simultaneously required.
As shown in the simple example above, even when the reactive component of the device impedance is neglected and when only eight devices are combined, the performance of corporate combiners in high power applications is quite limited in terms of bandwidth and power. There are two main difficulties here. One is the matching of the combined device impedance to a real resistive value and the second is the transformation of that real resistance to a 50-Ohm system, over a reasonable bandwidth and with practical insertion loss.
In terms of efficiency, due to its binary nature, a corporate scheme needs to combine either exactly 2 or 4 or 8 or 16 or 2n devices, without exception. For example, this means that even when the required power level calls for the use of only 20 active devices, actually 32 devices will be deployed instead. This leads to gross inefficiency as the number of devices to be used is large. In power amplifier circuits, inefficiency leads to more difficult thermal management and ultimately poor reliability. Again, this attribute of corporate-combined amplifiers is also not very desirable.
Also in wide band (for example, more than one octave) amplifiers, corporate power combine offers little defense against high harmonic contents especially when the second harmonic of an excitation tone falls within the bandwidth of interest. This again is another undesirable attribute of conventional power amplifier design because of the fact that second harmonics are the main contributors to the creation of the menacing third-order inter-modulation products. These inter-modulation products are very harmful to modern modulation schemes.
FIG. 3 shows another prior art of a solid-state amplifier in which the active devices are operated in a push-pull mode, within a corporate scheme. The push-pull configuration alleviates somewhat the impedance matching issue and allows second harmonic terminations by making use of the virtual grounds associated with the push-pull topology. However, the prior art shows in FIG. 3 is still hampered by the matching and impedance transformation issues, by the insertion loss issue and by the inefficiency issue.
Prior art power amplifiers are discussed in the following papers authored by two of the present inventors: “Low-loss, Planar Monolithic Baluns for K/Ka-Band Applications”, James Schellenberg and Ky-Hien Do, 1999 Microwave Theory and Techniques Symposium MTT-S, Anaheim, Calif., US and “A Push-Pull Power MMIC Operating at K/Ka-Band Frequencies”, James Schellenberg and Ky-Hien Do, 1999 Microwave Theory and Techniques Symposium MTT-S, Anaheim, Calif., US.